Not applicable.
Not applicable.
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to enhancements in cutter elements and in manufacturing techniques for cutter elements and rolling cone bits.
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or xe2x80x9cgagexe2x80x9d of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as xe2x80x9cTCIxe2x80x9d bits, while those having teeth formed from the cone material are commonly known as xe2x80x9csteel tooth bits.xe2x80x9d In each instance, the cutter elements on the rotating cutters breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a xe2x80x9ctripxe2x80x9d of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its ability to xe2x80x9chold gagexe2x80x9d (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (xe2x80x9cROPxe2x80x9d), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements (both steel teeth and tungsten carbide inserts) upon the cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.
The inserts in TCI bits are typically inserted in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, loss of cone material that otherwise provides protection for seals, and further results in imbalance of loads on the bit that may cause premature failure of the bit.
In addition to the heel row inserts, conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but orientated and sized in such a manner so as to cut the corner of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in circumferential rows disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.
One problem with conventional bit designs employing circumferential rows of spaced-apart inserts is that the discontinuous distribution of inserts allows severe wear to take place in the exposed region of the cone cutters between the individual inserts. Because the portion of the insert that is retained in the cone material is relatively small with conventional inserts having cylindrical bases, loss of adjacent cone material is a significant concern. This issue is particularly problematic in bits used in hard formations. As interstitial cone material is worn or eroded away from the regions between the inserts, the cone may lose its ability to absorb impact which, in turn, may lead to insert loss. Loss of inserts may both decrease ROP, and also lead to further erosion of the steel cone and loss of still additional inserts.
An additional design concern with TCI bits arises from the relatively small size of the heel row inserts. Generally, it would be desirable to include in the heel surface inserts having a relatively large diameter, and to provide the bit with a large number of such heel row inserts; however, the space available for inserts in the heel surface of the cone is severely limited due to the size and number of inserts placed in the gage row of the cone. The presence of the relatively large gage row inserts limits the size and the number of heel row inserts that can be retained in the adjacent heel surface. Because the heel row inserts on such conventional bits must therefore be relatively small in size and number, they do not offer the desired optimum protection against wear. In addition, the relatively small heel row inserts on conventional bits have other limitations: (a) they offer low strength against breakage/chipping caused by impact; (2) they must endure high contact stress while cutting formation material; (3) they possess relatively low capacity for heat dissipation. These factors contribute substantially to the failure modes of conventional rolling cone bits.
Accordingly, there remains a need in the art for a drill bit and cutting structure that are more durable than those conventionally known and that will retain inserts and cone material for longer periods so as to yield acceptable ROP""s and an increase in the footage drilled while maintaining a full gage borehole.
Preferred embodiments of the invention are disclosed that provide an earth boring bit having enhancements in cutter element design and in manufacturing techniques that provide the potential for increased bit life and footage drilled at full gage, as compared with similar bits of conventional technology. The embodiments disclosed include arcuate-shaped inserts of various arcuate lengths made through a conventional manufacturing process such as HIP. These inserts are disposed within a groove formed in the cone cutter of the rolling cone bit. Such inserts may also be placed in grooves formed elsewhere on the bit. The inserts include a plurality of spaced apart stress relief discontinuities, such as notches or grooves, such that, when the arcuate insert (including a full ring-shaped insert) is press fit within the cone groove, the insert will fragment at predetermined locations into a number of smaller, arcuate-shaped inserts. In certain embodiments, the arcuate-shaped inserts are disposed in an end-to-end relationship within the groove in the cone and substantially fill the cone groove.
The arcuate inserts may be disposed in the back face, the heel surface or any other surface of the rolling cone cutter, including the general conical surface that retains inserts that are employed in attacking the corner or the bottom of the borehole. Arcuate inserts, including full ring-shaped inserts, may be applied in multiple locations on the same cone cutter. Further, depending upon the cutting duty to be imposed on the inserts, as well as the expected formation material, the arcuate elements may have cutting surfaces configured in a variety of ways, including grooves having both positive and negative back rack, as well as intersecting grooves, that form cutting edges. Additionally, the cutting surfaces may have a variety of protrusions or recesses shaped to provide the cutting action desired.
The preferred embodiments disclosed contemplate the use of different materials to form the arcuate-shaped inserts or portions thereof. For example, the cutting surface may be made of a hard, wear resistant material, while the portion of the insert retained in the cone groove or channel may be made of a tougher material that is less likely to fracture than if it were made of the same hard, wear resistant material as the cutting surface. Similarly, the cutting surface may have different regions or segments made of different materials. For example, the radially outermost region of the cutting surface may be made of a harder more wear resistant material, while the innermost region is made of a tougher less brittle material.
The stress relief discontinuities may include grooves of various cross sections, such as v-shaped or u-shaped, or square grooves. Such notches or grooves may be unidirectional, meaning extending in only a straight line, or they may be 3-dimensional in that they have portions extending in a first direction and portions that deviate from that first direction and extend into a different plane.
The embodiments disclosed further include a variety of features enhancing the inserts ability to resist rotational movement within the cone groove, such features including non-circular inner surfaces or outer surfaces, tabs, concavities, edges or flats formed on the inner or outer surfaces of the arcuate-shaped inserts that engage similarly shaped features in the cone groove. Engaging pegs and corresponding recesses in the inserts and cone groove may also be employed
Providing arcuate inserts in a groove about the entire cone or the major portion thereof, and manufacturing the inserts of extremely hard or durable materials as permitted by HIP technology, overcomes certain problems associated with conventional bits. Specifically, the arcuate inserts extending about the cone surface eliminates the areas in conventional bits between the cylindrical-based inserts that were vulnerable to erosion and premature wear. The bits and rolling cone cutters disclosed in the present application better protect the material between the extending protrusions of the cutting surface and better protect against insert breakage and loss. Further, in the embodiments herein disclosed, the heat generated by the cutting surface is better able to be dissipated by virtue of the greater size of the arcuate insert as compared to the conventional, cylindrical-based inserts. This permits the arcuate inserts to retain their desirable material characteristics for a longer period of time whereas with conventional bits, the extreme heat could degrade or deteriorate the insert material.
The bits, rolling cone cutters, and arcuate inserts described herein provide opportunities for greater improvement in cutter element life and thus bit durability and ROP potential. These and various other characteristics and advantages will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.